Seasonal modulation of the spring-neap response of Yangtze Estuary turbidity maximum: Movement, amplitude, and phase lag via remote sensing

Estuarine turbidity maximum zones (ETMs) exhibit significant spring-neap and seasonal variations. Quantifying these dynamics from sparse, long-term remote sensing data is crucial for understanding ETM evolution. Using hourly GOCI satellite imagery (2011–2021) from the Yangtze Estuary, we applied Lomb-Scargle periodogram and phase-folded methods to extract the horizontal movement characteristics (amplitude and phase) of the ETM, tracked via 150, 100, and 50 mg/L SSC isolines. We found that the ETM extent has obvious spring-neap variations that lag the tidal cycle. Notably, high-SSC regions showed heightened sensitivity to tidal forcing: the 150 mg/L isoline had a larger amplitude (5.05 km) and shorter lag (1.42 days) compared to the 50 mg/L isoline (4.29 km amplitude, 1.99-day lag). Both amplitude and phase also exhibited significant seasonality. The phase lag proved highly sensitive to these seasonal changes: it was most pronounced in summer (a 2.44-day lag), decreased in spring (1.32 days) and autumn (0.65 days), and became a slight phase lead in winter (−0.13 days). These seasonal dynamics are linked to variations in sediment supply, waves, and river discharge. This study's approach provides a robust framework for extracting ETM spring-neap characteristics from sparse satellite data, clarifying multi-timescale ETM responses for similar estuarine studies.